Optical system and optical apparatus

ABSTRACT

An optical system includes a plurality of lenses, a diaphragm configured to adjust a light amount, and an optical element having a transmittance distribution. A transmittance of the optical element satisfies predetermined conditions. In addition, another predetermined condition is satisfied when the diaphragm is fully opened.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an optical system and an opticalapparatus.

Description of the Related Art

In general, the optical performance of the image pickup optical systemis evaluated based on the imaging performance of a focused object.However, depending on applications, an appearance of a defocused imagemay be an important evaluation factor for the optical performance of theimage pickup optical system. In particular, in an image pickup opticalsystem used for an image pickup apparatus, such as a film-based camera,a digital camera using a semiconductor image sensor, a video camera, anda TV camera, it is likely that the appearance of the defocused image isregarded as more important.

Japanese Patent Laid-Open No. (“JP”) 9-236740 proposes an image pickuplens system that includes an apodization (“APD”) filter located near adiaphragm. This lens system changes a length in at least one lens unit,and entirely moves to the object side in focusing from an infinityin-focus state to a short-distance in-focus state.

In general, in image pickup optical systems from a wide angel end to anintermediate telephoto end, the sagittal halo of an off-axis light fluxcauses an uneven intensity of the defocused image in the imageperiphery, and the ADP filter is effective to removing the sagittalhalo. However, the APD filter disclosed in JP 9-236740 is configured sothat its transmittance distribution gradually decreases as a distancefrom the optical axis increases in a direction perpendicular to theoptical axis and forms an approximately Gaussian distribution.Therefore, the transmission of light near the light flux center otherthan the sagittal halo also decreases. As a result, a light amount dropsdisadvantageously.

SUMMARY OF THE INVENTION

The present invention provides an optical system and an image pickupapparatus that can improve an appearance of a defocused image whilerestraining a drop of a light amount of a defocused image.

The optical system according to the present invention includes aplurality of lenses, a diaphragm configured to adjust a light amount,and an optical element having a transmittance distribution. Atransmittance of the optical element decreases as a distance increasesfrom an optical axis of the optical element in a radial direction in arange of r₁<r, and the following conditional expressions are satisfiedin a range of 0≦r≦r₁0.9≦T(r)/T ₀; and0.5≦r ₁ /r _(a),where r is a distance in the radial direction from the optical axis ofthe optical element, r₁ is a distance in the radial direction from theoptical axis to a position at which the transmittance of the opticalelement has a value of 90% of a maximum value, T(r) is the transmittanceat a position having the distance r, T₀ is the maximum value of thetransmittance, and r_(a) is an effective diameter of the opticalelement. The following conditional expression is satisfied:(OP2OP3)/2r _(a) ≦r ₂ /r _(a)≦OP1/r _(a)when the diaphragm is fully opened, r₂ is a distance in the radialdirection from the optical axis to a position at which the transmittanceof the optical element has a value of 50% of the maximum value. P1 is anend point in an incident area of a most off-axis light flux in theoptical element. P2 is a top end of the incident area. P3 is a bottomend of the incident area. OPi is a distance between the optical axis anda point corresponding to Pi (i=1, 2, 3) projected onto a planeperpendicular to the optical axis.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrate transmittance distributions of a transmittancedistribution element according to this embodiment and a generalapodization (APD) element.

FIG. 2 is a sectional view of an image pickup optical system accordingto a first embodiment of the present invention.

FIG. 3 is a view illustrating incident areas of an on-axis light fluxand the most off-axis light flux on a transmittance distribution elementin the image pickup optical system illustrated in FIG. 2 according tothe first embodiment.

FIG. 4 is a view illustrating an intensity distribution of a defocusedimage formed by the image pickup optical system illustrated in FIG. 2according to the first embodiment.

FIG. 5 is a sectional view of an image pickup optical system accordingto a second embodiment of the present invention.

FIGS. 6A and 6B are views illustrating incident areas of an axial lightflux and the most off-axis light flux on a transmittance distributionelement in the image pickup optical system illustrated in FIG. 5according to the second embodiment.

FIG. 7 is a view illustrating an intensity distribution of a defocusedimage formed by the image pickup optical system illustrated in FIG. 5according to the second embodiment.

FIG. 8 is a sectional view of an image pickup optical system accordingto a third embodiment of the present invention.

FIG. 9 is a view illustrating incident areas of an axial light flux andthe most off-axis light flux on a transmittance distribution element inthe image pickup optical system illustrated in FIG. 8 according to thethird embodiment.

FIG. 10 is a view illustrating an intensity distribution of a defocusedimage formed by the image pickup optical system illustrated in FIG. 8according to the third embodiment.

FIG. 11 is a sectional view of an image pickup optical system accordingto a fourth embodiment of the present invention.

FIGS. 12A and 12B are views illustrating incident areas of an axiallight flux and the most off-axis light flux on a transmittancedistribution element in the image pickup optical system illustrated inFIG. 11 according to the fourth embodiment.

FIG. 13 is a view illustrating an intensity distribution of a defocusedimage formed by the image pickup optical system illustrated in FIG. 11according to the fourth embodiment.

FIG. 14 is a view illustrating transmittance distributions of thetransmittance distribution elements according to the first, second,third, and fourth embodiments of the present invention.

FIG. 15 is a block diagram of an image pickup system according to thisembodiment.

DESCRIPTION OF THE EMBODIMENTS

FIG. 15 is a block diagram of an image pickup system (optical apparatus)100 according to this embodiment. The image pickup system includes aninterchangeable lens (optical apparatus) 110, and an image pickupapparatus 120. The interchangeable lens 110 is interchangeably attachedto and detached from the image pickup apparatus 120. The interchangeablelens 110 includes an optical system according to this embodiment. Theimage pickup apparatus 120 is, for example, an image pickup apparatus,such as a film-based camera, a digital camera using a semiconductorimage sensor, a video camera, and a TV camera, and may be a single-lensreflex camera and a single-lens non-reflex camera. Since the presentinvention is applicable to a lens integrated type image pickupapparatus, the optical apparatus may serve as an image pickup apparatus.Alternatively, the optical system of this embodiment is applicable toanother optical apparatus, such as a microscope and a projection typedisplay apparatus. A description will now be given of an optical systemaccording to this embodiment using an image pickup optical systemprovided in the interchangeable lens 110 as an example.

The image pickup optical system is applicable to image pickup of athree-dimensional object. The “three-dimensional object,” as usedherein, is an object that includes a plurality of parts having differentdistances in the optical axis direction, and in particular an objecthaving a point distant from a focal plane of the image pickup opticalsystem by a depth of field or longer at the image pickup time.

At this time, a defocused image is formed on an image plane. When adiameter of the defocused image is larger than a radius of an imagecircle by about 1-2% or more, the defocused image can be recognized. The“image circle,” as used herein, means a circle in which light passingthrough an effective diameter of the lens forms an image. When theoptical system according to this embodiment is applied to the imagepickup optical system in a digital still camera or a video camera, theimage plane is an image pickup plane of a semiconductor image sensor(photoelectric conversion element) such as a CCD sensor or a CMOSsensor. When the optical system of this embodiment is applied to theimage pickup optical system in a film-based camera, the image pickupplane is a film plane. The radius of the image circle may be a maximumimage height of the image pickup plane or film plane in the image pickupapparatus.

The optical system of this embodiment includes a transmittancedistribution element that provides a transmittance distribution to lightof each angle of view, and is configured to change a light amountdistribution of the defocused image. Since the defocused image that hasa large light amount in the outer circumference has an intensifiedcontour, the transmittance of the periphery of the defocused image maybe lower than that of the central part. The following descriptionassumes that the transmittance distribution element has a centrallysymmetrical transmittance distribution, and the transmittance at theperiphery is lower than that of the center part.

The image pickup optical system includes a plurality of lenses, adiaphragm configured to adjust a light amount, and a transmittancedistribution element.

The transmittance distribution element is an optical element having atransmittance distribution, and is obtained by evaporating an absorptionmaterial or a reflection material onto a transparent glass flat plateand a lens surface so as to provide a predetermined transmittancedistribution, or by applying and exposing a photosensitive material soas to provide a predetermined density. A concave lens made of a lightabsorbing material (ND glass) may be used. The transmittancedistribution may be variable using an electro chromic material, etc.

FIG. 1 is a graph illustrating (a) a transmittance distribution of thetransmittance distribution element according to this embodiment, and (b)a transmittance distribution of a general APD element. In FIG. 1, anabscissa axis denotes a distance r from the optical axis in a radialdirection. The radial direction is a direction from the optical axis tothe outside on a plane perpendicular to the optical axis. The ordinateaxis denotes T(r)/T₀. T(r) represents the transmittance at a positionhaving a distance r, and T₀ is the maximum value of the transmittance(maximum transmittance) in the effective diameter. The effectivediameter is an aperture diameter of the optical element through whichlight can pass. In addition, r₁ is a distance in the radial directionfrom the optical axis to a position at which the transmittance of thetransmittance distribution element has a value of 90% of the maximumtransmittance, and r₂ is a distance in the radial direction from theoptical axis to a position at which the transmittance of thetransmittance distribution element has a value of 50% of the maximumtransmittance.

The image pickup optical system that includes the Gaussian distributiontype transmittance distribution element illustrated by (b) in FIG. 1near the diaphragm can remove the sagittal halo that causes the unevenintensity of the defocused image, and improve the appearance of thedefocused image. However, the Gaussian distribution type transmittancedistribution provides the transmittance reducing effect even to lightthat passes near the center and is irrelevant to the uneven intensity ofthe defocused image, decreasing the light amount of the defocused imageand reducing the resultant size of the defocused image.

Accordingly, this embodiment uses the transmittance distribution elementillustrated by (a) in FIG. 1 to improve the appearance of the defocusedimage while restraining the drop of the light amount near the center ofthe light flux.

In a general image pickup optical system, an off-axis light flux suffersvignetting. The “vignetting” means shielding of part of light. In theimage pickup optical system causing the vignetting, a light passing areain the diaphragm is different between the on-axis light flux and theoff-axis light flux, and the effect obtained by the transmittancedistribution element differs according to angles of view. In general,the off-axis light flux passes a narrower range than the on-axis lightflux, and thus the transmittance distribution adjusted to the on-axislight flux does not always fit the off-axis light flux. When thetransmittance distribution element is distant from the diaphragm, thecenter of the off-axis light flux shifts from the optical axis. As aresult, the transmittance of the off-axis light flux becomesasymmetrical when the transmittance distribution element has a centrallysymmetrical transmittance distribution.

This embodiment addresses the fact that a shape of the off-axis lightflux becomes upside down between a position on the object side of thediaphragm and a position on the image side of the diaphragm, andprovides at least one transmittance distribution element near thediaphragm. This configuration can equivalently achieve an approximatelycentrally symmetrical pupil transmittance distribution of the off-axislight flux. When the defocused image improving effect at the off-axisangle of view is regarded as important, the optical element having thetransmittance distribution is disposed near the diaphragm. Thereby, theAPD effect that is effective even to the off-axis light flux can beobtained.

Assume that r is a distance in the radial direction that isperpendicular to the optical axis of the transmittance distribution,from the center which the optical axis of the transmittance distributionelement passes, and r₁ is a value of r with which the transmittance ofthe transmittance distribution element has a value of 90% of the maximumtransmittance in the effective diameter. Then, the transmittancedecreases as a distance increases from the optical axis in the radialdirection in a range outside of r₁ (r₁<r). In addition, the followingconditional expression (1) is satisfied in another range (0≦r≦r₁):0.9≦T(r)/T ₀  (1)

Moreover, r₁ satisfies the following conditional expression (2).0.5≦r ₁ /r _(a)  (2)

Herein, T(r) is a transmittance at a position with the distance r fromthe center in the radial direction, T₀ is the maximum transmittance inthe effective diameter, and r_(a) is the effective diameter.

In order to improve the defocused image having a large light amount inthe periphery and the emphasized edge, it is necessary to make lower thetransmittance in the periphery of the light flux than that at thecentral part of the light flux. Unless this condition is satisfied, thelight amount in the periphery of the light flux is made larger than thatat the central part of the light flux, and the defocused image becomesdeteriorated because the edge is emphasized. Therefore, thetransmittance of the transmittance distribution element can decrease inthe periphery as the distance increases from the center in the radialdirection.

The conditional expressions (1) and (2) are relate to the shape of thetransmittance distribution of the transmittance distribution element. Ina certain range from the center, a light amount taken by the lens systemcan be increased without reducing the transmittance. At the central partof the light flux, the transmittance may be constant in a predeterminedrange. When the transmittance distribution element is made of theelectro chromic material, it is difficult to smoothly change thetransmittance distribution. Therefore, the transmittance may be changedstepwise.

When the value is lower than the lower limit of the conditionalexpression (1), the transmittance becomes too low near the center of thetransmittance distribution element, and a light amount of the defocusedimage formed near the center on the image plane becomes lower. When thevalue is lower than the lower limit of the conditional expression (2),an area becomes too narrow, in which the transmittance is high near thecenter of the transmittance distribution element, and a light amountdrops near the center of the defocused image. Due to the manufacturescattering, the point that provides the highest transmittance may shiftfrom the optical axis or the transmittance distribution may becomeuneven. In these cases, unless the transmittance error falls within arange from 5% to 10%, the defocused image becomes deteriorated.

A description will now be given of suitable conditions for thisembodiment.

Assume that when the diaphragm is fully opened, r₂ is a distance in theradial direction from the optical axis to a position at which thetransmittance of the transmittance distribution element has a value of50% of the maximum transmittance T₀. O is the optical axis or the centerof the transmittance distribution element. In the transmittancedistribution element, P1 is an end point of an incident area of the mostoff-axis light flux, P2 is a top end of the incident area, and P3 is abottom end of the incident area. Then, the following conditionalexpression may be satisfied:(OP2+OP3)/2r _(a) ≦r ₂ /r _(a)≦OP1/r _(a)  (3)

Herein, OPi is a distance between a point corresponding to the point Oand a point corresponding to Pi (i=1, 2, 3) projected onto a planeperpendicular to the optical axis.

The conditional expression (3) relates to the shape of the transmittancedistribution of the transmittance distribution element. When the valueis lower than the lower limit of the conditional expression (3), an areabecomes too wide, in which the transmittance becomes lower than 50% ofthe maximum transmittance T₀. This configuration removes the light otherthan the sagittal halo component that causes the uneven intensity of thedefocused image, and the light amount of the defocus image becomesexcessively low. When the value exceeds the upper limit, the areabecomes too narrow, in which the transmittance becomes lower than 50% ofthe maximum transmittance T₀. Thereby, the defocused image improvingeffect by the transmittance distribution element reduces, and thedefocused image has the emphasized edge.

In focusing on infinity, the following conditional expression (4) may besatisfied:10 mm≦f/Fno≦70 mm  (4a)

where f is a focal length of the image pickup optical system, and Fno isan open aperture value.

The conditional expression (4) relates to an incident pupil diameter ofthe image pickup lens. When the value is lower than the lower limit ofthe conditional expression (4), an area in which each defocused imageoccupies becomes too narrow on the image pickup plane. Then, thetransmittance distribution given to the defocused image becomes toonarrow on the image pickup plane, and the defocused image improvingeffect by the transmittance distribution element reduces. Since thedefocused image is originally small and thus the deterioration of thedefocused image is less likely to stand out in the image pickup. Whenthe value exceeds the upper limit of the conditional expression (4), thedefocused image becomes large and the area in which each defocused imageoccupies becomes excessively large on the image pickup plane. While thedefocused image having the emphasized edge is formed due to theaberration of the image pickup optical system, the influence of theaberration on the light amount distribution of the defocused imagebecomes smaller in the large defocused image, in which the value ishigher than the upper limit of the conditional expression (4), and thedefocused image is less likely to stand out in the image pickup.

The conditional expression (4) may be replaced with the followingconditional expression:12 mm≦f/Fno≦70 mm  (4a)

The following conditional expression (5) may be satisfied:9°≦ω≦45°  (5)

where ω is half an angle of view of the image pickup lens.

The conditional expression (5) relates to half an angle of view of theimage pickup lens. While the defocused image having the emphasized edgeis formed due to the aberration of the image pickup optical system, adesigning effort can restrain the aberration that deteriorates the lightamount distribution of the defocused image in the image pickup opticalsystem in which the value is lower than the lower limit of theconditional expression (5). Therefore, the defocused image is lesslikely to stand out and the effect of the transmittance distributionelement reduces. When the value satisfies the lower limit value of theconditional expression (5), the background is likely to cause dot-shapedor thin-line-shaped light sources or objects, because of the backgroundcompression effect by the perspective of the image pickup lens. In suchan object, the contour of the defocused image is likely to stand out,and the effect of the transmittance distribution element becomes moreeffective. When the value exceeds the upper limit of the conditionalexpression (5), the vignetting for the off-axis light flux becomesworse. Then, it becomes difficult to provide defocused image improvingeffect equally to the on-axis light flux and the off-axis light flux.

In the transmittance distribution element, the following conditionalexpression may be satisfied:{(Tmax−Tmin)/Tmax}×100(%)≦20(%)

where Tmax is the maximum value of the transmittance and Tmin is aminimum value of the transmittance in a wavelength range from 430 nm to700 nm. This condition relates to a wavelength dispersion of thetransmittance. Unless this condition is satisfied, the periphery of thedefocused image becomes colored and the defocused image improving effectreduces.

The image pickup optical systems of respective embodiments areillustrated in FIGS. 2, 5, 8, and 11. In these figures, referencenumeral 11 represents the on-axis light flux, and reference numeral 12represents the most off-axis light flux. The “off-axis light flux” is alight flux that forms an image outside of the optical axis, and the“most off-axis light flux” is a light flux that forms an image at theoutermost part on the image pickup plane. Each figure representativelyillustrates a light flux incident on the image pickup optical systemfrom the lower side of optical axis. SP denotes the diaphragm, and IPdenotes the image plane. F1 and F2 denote the transmittance distributionelements.

When the diaphragm is fully opened, the vignetting states of the on-axislight flux and the most off-axis light flux on the transmittancedistribution elements are illustrated in FIGS. 3, 6A, 6B, 9, 12A, and12B. FIGS. 3, 6A, 9, and 12A correspond to a transmittance distributionelement F1, and FIGS. 6B and 12B correspond to a transmittancedistribution element F2. In each figure, reference numeral 20 denotes aneffective diameter of the transmittance distribution element, referencenumeral 21 denotes an incident area of the on-axis light flux, andreference numeral 22 denotes an incident area of the most off-axis lightflux. O denotes a center position of the transmittance distributionelement or the position of the optical axis. P1 is an end point of theincident area 22. P2 is an upper end of the incident area 22, and P3 isa lower end of the incident area 22.

Intensity distributions of the defocused images formed by the imagepickup optical systems according to respective embodiments on the imagepickup plane are illustrated in FIGS. 4, 7, 10, and 13. The abscissaaxis denotes the position R, and the ordinate axis denotes the intensityI(R) at the position R. As illustrated, reference numeral 31 denotes theintensity distribution of the defocused image at the on-axis angle ofview when the transmittance distribution element is not provided.Reference numeral 32 denotes the intensity distribution of the defocusedimage at the on-axis angle of view when the transmittance distributionelement is provided. Reference numeral 33 denotes the intensitydistribution of the defocused image at the off-axis angle of view whenthe transmittance distribution element is not provided. Referencenumeral 34 denotes the intensity distribution of the defocused image atthe off-axis angle of view when the transmittance distribution elementis provided. I₀ denotes the intensity at the position on the imagepickup plane which the principal ray enters. The intensity distributionof the defocused image is evaluated on the section that is perpendicularto the meridional plane and passes the incident position of theprincipal ray.

FIG. 14 illustrates the transmittance of the transmittance distributionelement of each embodiment. The abscissa axis denotes r/r_(a), and theordinate axis denotes T(r)/T₀.

A detailed description will now be given of each embodiment of thepresent invention.

First Embodiment

FIG. 2 is a sectional view of an image pickup optical system accordingto a first embodiment. A transmittance distribution element F1 isdisposed on the front side (object side) near a diaphragm SP. Thetransmittance distribution element F1 can be formed by evaporating amaterial onto a transparent glass flat plate. The transmittancedistribution element F1 provides a pupil intensity distribution to thelight flux of all angles of view containing the on-axis light flux 11and the most off-axis light flux 12, improving the appearance of thedefocused image. In the standard lens, only one transmittancedistribution element may be sufficient. This configuration facilitatesmanufacturing instead of providing a plurality of transmittancedistribution elements.

FIG. 3 illustrates vignetting states of the on-axis light flux and themost off-axis light flux in the transmittance distribution element F1.As evident from FIG. 3, the vignetting of the most off-axis light fluxcan be seen. In order to provide the defocused image improving effecteven to the most off-axis light flux, the transmittance distributionelement F1 according to the first embodiment possesses the transmittancedistribution corresponding to reference numeral 41 illustrated in FIG.14.

FIG. 4 illustrates intensity distributions of the defocused image at theon-axis angle of view and at the off-axis angle of view with an imageheight of 18 mm and the axis formed on the image pickup plane. Thedefocused image is a defocused image of an object distant from the imagepickup plane by 235 f where f is a focal length of the image pickupoptical system, in focusing on a position distant from the image pickupplane by 50 f. As understood from FIG. 4, the defocused image having ahigh intensity in the periphery at the off-axis angle of view and thusthe emphasized edge is improved as the good defocused image in which theintensity in the periphery is mildly decreased by the transmittancedistribution 41. In addition, the APD effect appears at the off-axisangle of view corresponding to the image height of 18 mm, because theintensity at the periphery is decreased without a drop of the intensitynear the center.

Thus, the transmittance distribution element F1 according to the firstembodiment improves the appearance of the defocused image whilerestraining the drop of the light amount.

Second Embodiment

FIG. 5 is a sectional view of an image pickup optical system accordingto a second embodiment. A transmittance distribution element F1 isdisposed on the object side of the diaphragm, and the transmittancedistribution element F2 is disposed on the image side of the diaphragm.In order to equivalently achieve an approximately centrally symmetricalpupil transmittance distribution of the off-axis light flux, thetransmittance distribution elements are disposed at two positions(before and after the diaphragm) so that the shape of the off-axis lightflux becomes approximately centrally symmetrical. The transmittancedistribution element is formed on the lens surface having a curvature sothat the lens surface has a transmittance distribution. In other words,the transmittance distribution element F1 is formed on the second lenssurface, and the transmittance distribution element F2 is formed on thefinal lens surface. The transmittance distribution elements F1 and F2provide the pupil intensity distribution to the light fluxes of allangles of view containing the on-axis light flux 11 and the mostoff-axis light flux 12, improving the appearance of the defocused image.

FIGS. 6A and 6B illustrate vignetting states of the on-axis light fluxand the most off-axis light flux in the transmittance distributionelement. As evident from FIGS. 6A and 6B, the vignetting of the off-axislight flux is seen. In order to provide the defocused image improvingeffect even to the most off-axis light flux, the transmittancedistribution elements F1 and F2 according to the second embodiment havethe transmittance distribution corresponding to reference numeral 42illustrated in FIG. 14.

FIG. 7 illustrates intensity distributions of the defocused image at theon-axis angle of view and at the off-axis angle of view with an imageheight of 18 mm and the axis formed on the image pickup plane. Assumethat f is a focal length of the image pickup optical system. Then, thedefocused image is a defocused image of an object distant from the imagepickup plane by 235 f in focusing on a position distant from the imagepickup plane by 50 f. As evident from FIG. 7, the defocused image havinga high intensity in the periphery at the off-axis angle of view and thusthe emphasized edge is improved as the good defocused image in which theintensity in the periphery is decreased by the transmittancedistribution 42. In addition, the APD effect appears at the off-axisangle of view corresponding to the image height of 18 mm, because theintensity difference between the central part and the periphery isdecreased. The appearance of the defocused image of the off-axis lightflux according to this embodiment is more improved than that accordingto the first embodiment, since the defocused image improving effect tothe off-axis light flux is regarded as important and the transmittancedistribution element is located near the diaphragm. At this time, nointensity drop near the center at the on-axis angle of view.

Thus, the transmittance distribution elements F1 and F2 according to thesecond embodiment improve the appearance of the defocused image whilerestraining the drop of the light amount.

Third Embodiment

FIG. 8 is a sectional view of an image pickup optical system accordingto a third embodiment. A transmittance distribution element F1 isdisposed on the image side of the diaphragm SP. The transmittancedistribution element F1 can be formed by evaporating a material onto atransparent glass flat plate. The transmittance distribution element F1can provide a pupil intensity distribution to the light flux of allangles of view containing the on-axis light flux 11 and the mostoff-axis light flux 12, improving the appearance of the defocused image.

FIG. 9 illustrates vignetting states of the on-axis light flux and themost off-axis light flux in the transmittance distribution element. Asevident from FIG. 9, the vignetting of the most off-axis light flux canbe seen. In order to provide the defocused image improving effect evento the most off-axis light flux, the transmittance distribution elementF1 according to the third embodiment is provided with the transmittancedistribution corresponding to reference numeral 43 illustrated in FIG.14.

FIG. 10 illustrates intensity distributions of the defocused image atthe on-axis angle of view and at the off-axis angle of view formed onthe image pickup plane. The defocused image is a defocused image of anobject distant from the image pickup plane by 90 f where f is a focallength of the image pickup optical system, in focusing on a positiondistant from the image pickup plane by 50 f. The image pickup opticalsystem according to the third embodiment can form comparatively gooddefocused image even without the transmittance distribution element, butcan further improve the defocused image at the on-axis angle of view andat the off-axis angle of view using the transmittance distribution 43,in which the intensity in the periphery is mildly decreased. At thistime, no intensity drops near the center.

Thus, the transmittance distribution element F3 according to the thirdembodiment improves the appearance of the defocused image whilerestraining the drop of the light amount.

Fourth Embodiment

FIG. 11 is a sectional view of an image pickup optical system accordingto a fourth embodiment. A transmittance distribution element F1 isdisposed on the object side of the diaphragm, and the transmittancedistribution element F2 is disposed on the image side of the diaphragm.In order to equivalently achieve an approximately centrally symmetricalpupil transmittance distribution of the off-axis light flux, thetransmittance distribution elements are disposed at two positions(before and after the diaphragm) so that the off-axis light flux becomesapproximately centrally symmetrical. The transmittance distributionelement is formed on the lens surface having a curvature so that thelens surface has a transmittance distribution. The transmittancedistribution elements F1 and F2 provide the pupil intensity distributionto the light fluxes of all angles of view containing the on-axis lightflux 11 and the most off-axis light flux 12, improving the appearance ofthe defocused image.

FIGS. 12A and 12B illustrate vignetting states of the on-axis light fluxand the most off-axis light flux in the transmittance distributionelements. As evident from FIGS. 12A and 12B, the vignetting of theoff-axis light flux can be seen. In order to provide the defocused imageimproving effect even to the most off-axis light flux, the transmittancedistribution elements F1 and F2 according to the fourth embodimentpossess the transmittance distribution corresponding to referencenumeral 44 illustrated in FIG. 14.

FIG. 13 illustrates intensity distributions of the defocused image atthe on-axis angle of view and at the off-axis angle of view formed onthe image pickup plane. Assume that f is a focal length of the imagepickup optical system. Then, the defocused image is a defocused image ofan infinity object in focusing on a position distant from the imagepickup plane by 16 f. As evident from FIG. 13, the defocused imagehaving a high intensity in the periphery at the off-axis angle of viewand thus the emphasized edge is improved as the good defocused image inwhich the intensity in the periphery is mildly decreased by thetransmittance distribution 44. At this time, no drop of intensity nearthe center occurs at the on-axis angle of view.

Thus, the transmittance distribution elements F1 and F2 according to thefourth embodiment improve the appearance of the defocused image whilerestraining the drop of the light amount.

Next follows numerical examples 1 to 4 corresponding to the first tofourth embodiments. In each numerical example, “r” denotes a radius ofcurvature (mm) of an i-th plane from the object side, and “d” denotes asurface distance (mm) along the optical axis between the i-th surfaceand the (i+1)-th surface from the object side. “nd” and “vd” denote arefractive index and an Abbe number of the d-line of the opticalelement. The focal length f (mm), the F-number Fno, and the angle ofview ω (degree) are values when the infinity object is focused. BFdenotes a back focus, and the lens overall length is a distance from thefirst plane to the image plane. “*” after the surface number denotes anaspheric surface. An aspheric shape is expressed by a displacementamount X from a surface vertex in the optical axis direction and aheight h from the optical axis in the direction perpendicular to theoptical axis, a paraxial radius of curvature r, a conical constant K,and aspheric coefficients A4, A6, A8, and A10:

The lens unit includes at least one lens. When the lens unit includes aplurality of lenses, one movable lens unit is a unit that integrallymoves during zooming or focusing.

${X(h)} = {\frac{h^{2}/r}{1 + \sqrt{1 - {\left( {1 + K} \right)\left( {h/r} \right)^{2}}}} + {A\; 4h^{4}} + {A\; 6h^{6}} + {A\; 8h^{8}} + {A\; 10h^{10}}}$

“e±Z” means “10^(±Z).”

Numerical Example 1

Unit: mm Surface data Surface effective No. r d nd νd diameter  1 ∞ 2.0042.78  2 55.776 5.50 1.77250 49.6 38.04  3 359.778 2.80 36.09  4 ∞ −1.6033.81  5 25.999 5.00 1.83481 42.7 31.97  6 42.223 1.20 29.70  7 57.6412.00 1.64769 33.8 29.56  8 17.674 8.40 25.24  9 ∞ 0.50 1.51633 64.124.55 10 ∞ 0.10 24.55 11(diaphragm) ∞ 6.50 24.55 12 −19.386 2.00 1.8051825.4 24.15 13 257.965 6.50 1.75700 47.8 28.14 14 −38.043 0.20 29.83 15−71.655 4.50 1.88300 40.8 30.67 16 −30.457 −0.80 31.22 17 ∞ 1.00 30.1418 123.258 3.00 1.80400 46.6 29.66 19 −104.699 0.00 30.02 20 ∞ 0.0030.32 21 ∞ (variable) 30.32 image plane ∞ various data zooming ratio1.00 focal length 52.46 Fno 1.48 angle of view 22.41 image height 21.64lens overall length 88.50 BF 39.70 d21 39.70 incident pupil position30.74 exit pupil position −34.08 front principal position (FPP) 45.90back principal position (BPP) −12.76 zoom lens unit data starting focallens overall Unit plane length length FPP BPP 1 1 52.46 8.80 45.90−12.76 single lens data lens starting plane focal length 1 1 84.78 2 571.09 3 7 −40.14 4 9 0.00 5 12 −22.32 6 13 44.21 7 15 57.07 8 18 70.83

Numerical Example 2

Unit: mm Surface data Surface effective No. r d nd νd diameter  1 ∞ 2.0042.78  2 57.520 5.50 1.77250 49.6 38.04  3 395.282 2.80 36.09  4 ∞ −1.6033.81  5 25.921 5.00 1.83481 42.7 31.97  6 43.002 1.20 29.70  7 59.0162.00 1.64769 33.8 29.56  8 17.697 9.00 25.24  9(diaphragm) ∞ 6.50 24.5510 −19.448 2.00 1.80518 25.4 24.15 11 280.464 6.50 1.75700 47.8 28.14 12−37.481 0.20 29.83 13 −74.530 4.50 1.88300 40.8 30.67 14 −30.712 −0.8031.22 15 ∞ 1.00 30.14 16 124.396 3.00 1.80400 46.6 29.66 17 −111.4540.00 30.02 18 ∞ 0.00 30.32 19 ∞ (variable) 30.32 image plane ∞ variousdata zooming ratio 1.00 focal length 52.50 Fno 1.49 angle of view 22.40image height 21.64 lens overall length 88.49 BF 39.69 d19 39.69 incidentpupil position 30.97 exit pupil position −33.78 front principal position45.96 back principal position −12.81 zoom lens unit data starting focallens overall Unit plane length length FPP BPP 1 1 52.50 48.80 45.96−12.81 single lens data lens starting plane focal length 1 1 86.53 2 568.99 3 7 −39.78 4 10 −22.52 5 11 44.06 6 13 56.44 7 16 73.53

Numerical Example 3

Unit: mm Surface data Surface effective No. r d nd νd diameter  1104.460 9.00 1.48749 70.2 67.53  2 −300.232 0.50 66.86  3 51.920 9.501.49700 81.5 59.38  4 243.875 3.00 58.00  5 −610.337 3.50 1.83400 37.257.41  6 93.667 2.50 53.72  7 61.059 8.00 1.49700 81.5 52.23  8 −583.7530.50 51.39  9 29.125 3.20 1.71736 29.5 42.31 10 24.344 12.50  38.0111(diaphragm) ∞ (variable) 35.56 12 −1837.096 4.50 1.84666 23.9 33.81 13−55.859 2.00 1.72000 50.2 33.05 14 41.346 4.00 29.79 15 ∞ 0.10 29.51 16∞ 0.40 1.51633 64.1 29.49 17 ∞ (variable) 29.43 18 −31.045 2.50 1.7407727.8 26.05 19 175.679 8.50 1.77250 49.6 28.90 20 −40.350 0.50 32.19 21107.611 6.00 1.83400 37.2 35.44 22 −183.700 (variable) 36.03 image plane∞ various data zooming ratio 1.00 focal length 133.50 Fno 2.06 angle ofview 9.21 image height 21.64 lens overall length 155.41 BF 54.41 d112.47 d17 17.83 d22 54.41 incident pupil position 74.70 exit pupilposition −91.21 front principal position 85.82 back principal position−79.09 zoom lens unit data starting focal lens overall Unit plane lengthlength FPP BPP 1 1 93.66 52.20 −7.82 −43.87 2 12 −64.15 11.00 3.66 −4.303 18 81.23 17.50 17.37 10.16 single lens data lens starting plane focallength 1 1 160.14 2 3 130.58 3 5 −97.15 4 7 111.68 5 9 −286.91 6 1267.97 7 13 −32.72 8 16 0.00 9 18 −35.43 10 19 43.22 11 21 82.14

Numerical Example 4

Unit: mm Surface data Surface effective No. r d nd νd diameter  1 ∞ 1.5054.03  2 79.773 2.00 1.60311 60.6 46.37  3 28.865 8.00 39.23  4 117.2124.00 1.77250 49.6 37.77  5 −212.879 (variable) 36.80  6 84.814 1.501.48749 70.2 24.71  7 19.679 10.00  21.16  8 22.595 3.50 1.91082 35.314.46  9 −45.147 1.00 1.73800 32.3 13.37 10 26.917 3.50 12.8711(diaphragm) ∞ (variable) 12.80 12 111.141 1.50 1.72916 54.7 12.69 13−73.101 (variable) 12.63 14 −13.184 1.50 1.74000 28.3 12.65 15 −132.8294.50 1.69680 55.5 15.15 16 −17.844 0.80 17.62 17* −53.671 3.20 1.5831359.4 19.54 18 −18.948 0.00 20.58 19 ∞ (variable) 22.26 image plane ∞Aspheric data 17-th surface K = 0.00000e+000 A4 = −2.50000e−005 A6 =4.20000e−008 A8 = −6.00000e−010 A10 = 2.00000e−012 various data zoomingratio 1.00 focal length 28.50 Fno 2.85 angle of view 37.20 image height21.64 lens overall length 99.00 BF 38.00 d5 7.00 d11 4.00 d13 3.50 d1938.00 incident pupil position 28.63 exit pupil position −29.20 frontprincipal position 45.04 back principal position 9.50 zoom lens unitdata starting focal lens overall Unit plane length length FPP BPP 1 1−528.68 15.50 −39.98 −57.63 2 6 374.88 19.50 48.86 38.18 3 12 60.69 1.500.53 −0.35 4 14 61.47 10.00 17.07 15.27 single lens data lens startingplane focal length 1 1 −76.12 2 4 98.37 3 6 −52.96 4 8 16.95 5 9 −22.726 12 60.69 7 14 −19.88 8 15 29.12 9 17 48.58

Table 1 summarizes numerical values corresponding to transmittanceT(0.5) located at a position with a distance from the center of thetransmittance distribution element in the radial direction which is 50%of the effective diameter of the transmittance distribution element.Table 2 summarizes numerical values corresponding to the conditionalexpressions (2) to (5) in each numerical examples. Table 3 summarizesnumerical values corresponding to the upper limit value and the lowerlimit value of the conditional expression (3).

TABLE 1 Numerical Numerical Numerical Numerical Example 1 Example 2Example 3 Example 4 T(0.5) 0.91 0.98 0.93 0.98

TABLE 2 Numerical Numerical Numerical Numerical Example 1 Example 2Example 3 Example 4 Conditional 0.51 0.66 0.53 0.63 Expression (2)Conditional 0.65 0.85 0.70 0.80 Expression (3) Conditional 35.4 35.264.8 10.0 Expression (4) Conditional 22.4 22.4 9.2 37.2 Expression (5)

TABLE 3 Num. Numerical Num. Numerical Ex. 1 Example 2 Ex. 3 Example 4 F1F1 F2 F1 F1 F2 OP1/r_(a) 0.77 1.00 1.00 0.87 0.99 1.00 OP2/r_(a) 0.310.34 1.00 0.49 0.16 0.97 OP3/r_(a) 0.35 1.00 0.43 0.46 1.00 0.26 (OP2 +OP3)/2r_(a) 0.33 0.67 0.72 0.48 0.58 0.62

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2015-021965, filed Feb. 6, 2015, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An optical system comprising: a plurality oflenses; a diaphragm configured to adjust a light amount; and an opticalelement having a transmittance distribution, wherein a transmittance ofthe optical element decreases as a distance increases from an opticalaxis of the optical element in a radial direction in a range of r₁<r,and the following conditional expressions are satisfied in a range of0.9≦T(r)/T ₀; and0.5≦r ₁ /r _(a), where r is a distance in the radial direction from theoptical axis, r₁ is a distance in the radial direction from the opticalaxis to a position at which the transmittance of the optical element hasa value of 90% of a maximum value, T(r) is the transmittance at aposition having the distance r, T₀ is the maximum value of thetransmittance, and r_(a) is an effective diameter of the opticalelement, and wherein the following conditional expression is satisfied:(OP2+OP3)/2r _(a) ≦r ₂ /r _(a)≦OP1/r _(a) where the diaphragm is fullyopened, r₂ is a distance in the radial direction from the optical axisto a position at which the transmittance of the optical element has avalue of 50% of the maximum value, P1 is an end point in an incidentarea of a most off-axis light flux in the optical element, P2 is a topend of the incident area, P3 is a bottom end of the incident area, andOPi is a distance between the optical axis and a point corresponding toPi (i=1, 2, 3) projected onto a plane perpendicular to the optical axis.2. The optical system according to claim 1, wherein in focusing oninfinity, the following conditional expression is satisfied:10 mm≦f/Fno≦70 mm where f is a focal length of the optical system, andFno is an open aperture value.
 3. The optical system according to claim1, wherein the following conditional expression is satisfied:9°≦ω≦45°  (5) where ω is half an angle of view of the optical system. 4.The optical system according to claim 1, wherein the followingconditional expression is satisfied:{(Tmax−Tmin)/Tmax}×100(%)≦20(%) where Tmax is the maximum value of thetransmittance and Tmin is a minimum value of the transmittance in awavelength range from 430 nm to 700 nm.
 5. The optical system accordingto claim 1, wherein the optical element is disposed near the diaphragm.6. The optical system according to claim 1, wherein two optical elementsare disposed before and after the diaphragm.
 7. An optical apparatuscomprising an optical system, wherein the optical system includes: aplurality of lenses; a diaphragm configured to adjust a light amount;and an optical element having a transmittance distribution, wherein atransmittance of the optical element decreases as a distance increasesfrom an optical axis of the optical element in a radial direction in arange of r₁<r, and the following conditional expressions are satisfiedin a range of0.9≦T(r)/T ₀; and0.5≦r ₁ /r _(a), where r is a distance in the radial direction from theoptical axis, r₁ is a distance in the radial direction from the opticalaxis to a position at which the transmittance of the optical element hasa value of 90% of a maximum value, T(r) is the transmittance at aposition having the distance r, T₀ is the maximum value of thetransmittance, and r_(a) is an effective diameter of the opticalelement, and wherein the following conditional expression is satisfied:(OP2+OP3)/2r _(a) ≦r ₂ /r _(a)≦OP1/r _(a) where the diaphragm is fullyopened, r₂ is a distance in the radial direction from the optical axisto a position at which the transmittance of the optical element has avalue of 50% of the maximum value, P1 is an end point in an incidentarea of a most off-axis light flux in the optical element, P2 is a topend of the incident area, P3 is a bottom end of the incident area, andOPi is a distance between the optical axis and a point corresponding toPi (i=1, 2, 3) projected onto a plane perpendicular to the optical axis.